Chemical disposition studies are designed to provide both basic and applied knowledge of the fate and toxicity of selected chemicals in support of the National Toxicology Program. As such, these studies are designed to investigate both the effect of chemical structure and properties on acute and chronic toxicity and, whenever possible, investigate the effect of these factors on mechanisms of chemical toxicity and carcinogenicity. Recent studies in chemical disposition have addressed species-specific variation in the metabolism and clearance of ethyl carbamate and have provided data to document the significantly more rapid metabolism of this chemical by mice than rats. This data has further demonstrated that most studies of ethyl carbamate carcinogenicity have used doses which saturate metabolism and thus may not be optimum for extrapolation of animal data to predict human risks. Another study has addressed the fate of a preparation of polyvinyl alcohol designed for use with a spermicide. This work has demonstrated that absorption of polyvinyl alcohol from the gastrointestinal tract is negligible, but absorption may be significant when this compound is applied intravaginally. Further, the absorbed polyvinyl alcohol appears to be retained in the liver and other tissues and only slowly eliminated. Current work in this group is also addressing the species-dependent toxicity of tris(2- chloroethyl) phosphate observed in chronic studies of this compound. This work has demonstrated that this compound is readily absorbed from the gastrointestinal tract, but is not excreted prior to metabolism. The metabolites, once formed, are rapidly excreted in urine. This work has further demonstrated that metabolism may be saturated by doses which result in chronic toxicity. Therefore, it appears that the toxicity of tris (2-chloroethyl) phosphate may be attributed to parent compound.